A wide variety of applications utilize quadrature signal processing to efficiently extract information from received signals. Such applications may include, but are not limited to, video communication and distributions systems, and wireless data and/or voice communications. Such applications may fall into a broad class of systems known as coherent communication systems. These systems typically preserve the phase of a received signal, and allow for the reliable extraction of any information encoded therein.
For coherent communications systems, the quadrature signal representation provides a convenient format for extracting phase information. Moreover, signals represented in the quadrature format allow for the unambiguous detection of positive and negative frequencies centered at baseband. Utilizing the quadrature format of the received signal can make frequency discriminations straightforward.
Techniques for converting a received signal into the quadrature format are known as quadrature sampling, or In-phase/Quadrature (IQ) sampling. Conventionally, this may be accomplished by first down-converting the bandpass signal centered on a carrier frequency to in-phase (I) and quadrature (Q) baseband signals centered at direct current (DC) (i.e. Zero-IF (intermediate frequency)), then sampling these signals with two separate I and Q analog to digital converters (ADCs), as depicted in FIG. 1A. Alternatively, one ADC may be used to sample sequentially I and Q at higher rate (a minimum of two times faster than in FIG. 1A, i.e. a minimum of four times the Nyquist rate), as shown in FIG. 1B. These methods may be referred to as “indirect quadrature sampling” since they involve a frequency translation step before I and Q sampling. On the other hand, a direct quadrature sampling refers to directly sampling signals without a conversion to Zero-IF.
FIG. 1A shows one example of conventional “indirect” quadrature sampling, which may include a local oscillator 118, a phase shifter 116, first and second multipliers 102 and 114, first and second low pass filters (LPFs) 104 and 112, and first and second ADCs 106 and 110. The outputs of the I and Q channels may be passed on to any processing device, for example, a digital signal processor (DSP) 108, or may be recorded digitally for subsequent processing.
FIG. 1B shows another example of a conventional indirect sampling architecture which may replace ADC 106 and ADC 110 with a switch 120 to sample both the I and Q channels sequentially so that only one ADC 122 is needed (albeit sampling at twice the rate) to produce the I and Q samples. Reducing the number of ADCs can improve the IQ matching and reduce the cost. Because the I and Q channels are “serially” sampled by switch 120, the resulting I and Q samples will be misaligned in time by approximately a half sample time, and will have to be temporally aligned subsequent to further processing.
FIG. 1C shows an example of a conventional quadrature sampling at IF frequencies. In this example, the input signal may initially be centered at an RF frequency of 476 MHz. The input signal may be down-converted to an IF signal centered at 4.9 MHz by using a signal multiplier 130 and a 471.1 MHz sinusoidal signal generated by local oscillator (LO) 135. Images in the frequency-shifted may be rejected by filtering with a band pass filter 140. The filtered IF signal may be sampled by ADC 145 using a clock rate with is 4×the IF center frequency (e.g., 19.6 MHz). A demultiplexer 150 may demultiplex the IF samples at twice the IF center frequency (e.g., 9.8 MHz). Each demultiplexed stream may then downconverted to baseband by multiplying (performing sign inversion) the IF signal at a 1×IF center frequency rate (e.g., 4.9 MHz).
The traditional techniques for accurately performing quadrature sampling may be limited to a narrow frequency range (e.g., on the order of one percent) around a single frequency corresponding to the system's sampling clock frequency. This limitation arises because the phase offset between the I and Q samples may drift away from 90 degrees as the frequency of the IF signal deviates from the sampling frequency. Moreover, traditional techniques fail to directly sample the RF input signal and typically require at least one frequency down-conversion step prior to performing the sampling.
Thus, these traditional techniques may not be appropriate for wide-band signals having large fractional bandwidths. Given the ever increasing expectations for improved systems' performance, the use of wide-band signals is becoming more and more commonplace. Conventional approaches that increase the frequency coverage of quadrature ADC's may result in more complex processing architectures. Such approaches may involve frequency conversion to baseband, often using a tunable local oscillator frequency which is appropriately mixed to provide a quadrature signal, and then subsequently sampled by the ADCs. Given the increased complexity of conventional techniques for quadrature sampling of wide-band signals, such implementations may be associated with increased cost, reduced reliability and reduced performance.
Accordingly, there is a need for direct quadrature sampling techniques which may be applicable to wide-band signals, and furthermore avoid the aforementioned issues of the conventional approaches.